Show warning message if you have merge lines using git-hooks(pre-commit). Details: https://attonblog.blogspot.jp/2017/11/git-pre-commit-hook.html

.gitconfig

[core]
        hooksPath = ~/.config/git/hooks

conflict.txt

<<<<<<< HEAD
hoge
=======
fuga
>>>>>>> another-branch

pre-commit

#!/bin/sh

FORBIDDEN='(<<<<<<<|=======|>>>>>>>)'
GREP_COLOR='4;5;37;41'
cd $PWD

git diff --cached --name-only | xargs -L1 grep --color --with-filename -n -E $FORBIDDEN
if [[ $? -eq 0 ]]; then
    echo "Merge lines was detected.\nPlease fix files, or run 'git commit --no-verify' to skip this check."
    exit 1
fi

result.txt

$ git commit -m 'perfect merge'
e:1:<<<<<<< HEAD
e:3:=======
e:5:>>>>>>> o
Merge lines was detected.
Please fix files, or run 'git commit --no-verify' to skip this check.
$ git status
On branch master
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

	modified:   hoge.txt